Abstract At high oxygen pressures ( P O 2 #62; 1 × 10 −3 Torr) and at fairly low temperature ( T = 400 K) the catalytic CO oxidation on Pd(110) exhibits kinetic oscillations which have been followed by means of measurements of the reaction rate and of the work function change. Under these conditions the kinetics of CO 2 formation develop pronounced hysteresis effects which differ significantly from those observed with Pt surfaces. The unusual features in the CO 2 hysteresis could be related to the formation of subsurface oxygen which has been demonstrated to take place under the same conditions. As subsurface oxygen decreases the catalytic activity of the surface, the characteristic hysteresis effects could be traced back to a slow filling and depleting of the subsurface oxygen reservoir under conditions of varying adsorbate coverages. A mechanism for the kinetic oscillations on Pd(110) is suggested in which the slow formation/removal of subsurface oxygen modulates the catalytic activity of the surface. The proposed mechanism is supported experimentally by the coincidence of the p O 2 , T conditions for kinetic oscillations with the conditions under which the characteristic features in the CO 2 hysteresis due to the formation of subsurface oxygen can be observed. The operation of a mechanism which is based on structural changes, e.g. a surface phase transition or reversible facetting, can be excluded on the basis of the property of the clean Pd(110) surface which does not reconstruct, as well as from the absence of kinetic oscillations in the low pressure regime ( p O 2 < 1 × 10 −3 Torr).